(1) Field of the Invention
This invention relates to a method of screen printing of surface layers on substrates from a paste formulated from an appropriate solid-binder-solvent-plasticizer combination. The printed pattern is converted by heat treatment to form thin surface layers useful as phosphors in light-emitting applications such as cathode ray screens. In particular, the paste formulation may be varied to produce printed patterns which are useful as conductor and interconnection means for printed circuits, as resistors, capacitors. inductors, or crossovers, and as sealing band patterns.
(2) Description of the Prior Art
The production of patterns of material on surfaces by means of forcing a semi-solid material held on a fine screen through a patterned opening closely adjacent to the surface is known as serigraphy or, more commonly, as silk screen printing. It has been used for many years as an economical means for producing patterns of materials commonly referred to as inks, pastes, creams, etc. which are then converted by drying, heat, or other means to a permanent pattern layer with the desired properties. In more recent times, silk as the fine screen material has been replaced by mesh screens woven from fine metal wires, and the process is referred to as simply screen printing. In the electrical and electronic industries, screen printing has been used to form conductive, resistive, insulating, and magnetic patterns, as well as combinations thereof. In general, the art of screen printing is highly dependent on the details of formulation of the ink etc. medium to be printed, hereinafter referred to as the paste as a generic term, and on the specific techniques of application of the paste to form the pattern. The success of the art of screen printing is also very dependent on the specific techniques employed, such as the size and texture of the screen or mesh, the thickness of the layer, the pressure and speed of the means of spreading and forcing the paste through the screen and the stencil openings which define the pattern, and a variety of other lesser features of the screening process.
The formulation of a screening paste requires that a uniform blending be made of the material which gives the paste its primary characteristic, such as a phosphorescent substance or phosphor, for example, which is generally a fine solid uniform powder, and the materials which provide the necessary properties of holding the powder together as a mass capable of being manipulated. The latter are generally liquids of an organic nature, and are from the classes of substances referred to as solvents, thinners, plasticizers, binders, leveling agents, etc. many of which are common to the paint and coatings art. The usual distinctions made among paints, inks, greases, pastes and the like deal with variations in their rheological properties; that is, their ability to flow. Such materials vary greatly in viscosity as well as functionality, and the term "paste" is generally reserved for compositions which possess not only a very high value of viscosity, i. e. are very thick, but also are non-Newtonian in their behaviour; the measured viscosity is a function of the rate of shear of the material being measured. Often, these materials are described as thixotropic: they undergo little or no flow or relaxation of shape while undisturbed, but flow like a liquid when mechanical stirring is employed. This property of flowing when a shearing stress is employed is very useful in pastes for screen printing, since it allows the paste to rest upon the fine mesh of the screen without leaking or oozing through until the printing tool, called the squeegee, pushes the paste along the screen and forces it through the screen to the stencil, where the paste is deposited on the adjacent surface in the pattern of the stencil. After the paste has been forced through the fine screen, its mechanical integrity and uniformity in the pattern layer are regenerated, and the pattern is retained as the layer is dried and treated, generally by heating, to remove the organic liquids and other residues and bring about the desired properties of the printed layer.
The formulations of pastes and inks useful in the screen printing of surface layers of phosphors are abundant in the literature. U.S. Pat. No. 5,478,381 to Okiwa et al shows an ink composition consisting of alkylamines and phosphate salts, a binder resin that is excellent in light-emitting characteristics. U.S. Pat. Nos. 5,132,045 and 5,277,840, both to Oska et al, disclose paste compositions of phosphor powders and acrylic polymer for screen printing applications. None of the cited patents reveal the detailed use of the specific materials butyl carbitol, polyvinylbutyral (PVB), or phthalate esters such as butyl benzyl phthalate (BBP) to formulate the pastes. U.S. Pat. No. 5,338,350 to Alexandres et al discloses the use of butyl carbitol to formulate ink for a metal oxide layer, but does not mention phosphor or other powders nor the use of PVB, BBP, or other organic materials.